I have a problem in using alpha value. I know how to set alpha value for background but I don't know how to set 20% alpha value for ImageButton.
You don't need a tutorial for this, just read the javadoc of the method, which says:
setAlpha() - Sets the opacity of the view. This is a value from 0 to 1, where 0 means the view is completely transparent and 1 means the view is completely opaque.
20% transparent then would mean a value of 0.2.